Just have the following phrase as a back-up plan: "oh, look I'm sorry, I'll have to call back, I just thought of something important I must do, sorry about this, okay bye now."

It's not a lie, you did think of something important you must do: you just thought that you have to call a different rep at HH.

If you don't, this is what might happen:

You: Hi, this is Mr. srodr and I would like to add a note to reservation # 123, for this evening, that I will be arriving late as my meeting with Queen E II is going into extra hours.

Rep: Ummm, okay . . . <long pause> . . . sir, I don't have a reservation # 132 . . . are you sure it's for the Hilton chain ?

You: <alarm bells ringing - remembering SLS's advice but not following through> . . . That's reservation # 123 and not # 132 . . . and yes it's for a Hilton property.

Rep: Oh, yes sir . . . <long pause . . . agent clicking furiously . . . you are getting nervous, thinking it's just the hour she has to change so what's with all the typing?> . . . Well, okay sir the change has been done . . . just to confirm you will be arriving at the Hilton London Ontario Canada on Dec 25, 2008 at 6 PM . . . thank you for calling HH . . . bye now . . . <click>.

You: Hello ! Hello !! London, England you poop . . . and it's for tonight, not next year . . . ahhhhhhh